What does HC3 mean in 'His HC3'?

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Summary:

My lecturer said it means: 3rd residue, Cter to Helix 'H' but that makes no sense.

Main Question or Discussion Point

My lecturer said it means: "3rd residue, C-terminal to Helix 'H'" but that makes no sense. If it's the 3rd residue then it can't be on the C-terminal, because the C-terminal is at the end of a massively long sequence of residues. Visa versa if it's on the C-terminal then it can't be the 3rd residue.
I tried asking him but all I got in response was a lot of hand waving and "don't worry about this, just learn the overall picture blah blah blah". But that's unacceptable, I found something that doesn't make sense to me and I deserve an answer if I am paying hundreds of thousands of dollars.
Googling turned up nothing. So I created an account here and am seeking your help.

A little background: His HC3 is a Histidine residue and forms one of the most important salt bridges when oxygen is removed from hemoglobin, it stabilises the "T" state of Hb. His HC3 forms a salt bridge with Asp FG1 (FG1 meaning: "1st residue, in the loop between helices 'F' and 'G'" which thankfully makes sense!)
 
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Answers and Replies

  • #2
Ygggdrasil
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Even though helix H does not occur at the N-terminus or C-terminus of the protein, it still does have a directionality to it. The N-terminal end of the helix is closer in primary sequence to the N-terminus of the protein and the C-terminal end of the helix is closer in primary sequence to the C-terminus of the protein (in the same way we can talk about the 5' end of a gene and the 3' end of a gene, even if the gene occurs in the middle of a chromosome, far away from the actual 5' and 3' ends of the DNA molecule in which the gene resides). So, another way of saying "3rd residue, Cter to Helix 'H'" is the third residue after the end of helix H (when numbering residues from N-ter to C-ter as is conventional).
 
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